As semiconductor devices continue to become more highly integrated, the cell size of semiconductor devices such as dynamic random access memory (DRAM) devices continues to be reduced. As a result, the effective area of the lower electrode of the cell capacitor in such devices also becomes reduced. Typically, a certain level of cell capacitance is required to ensure stable operation of the device. To secure sufficient cell capacitance in a small area, the capacitor dielectric layer may be formed of a material such as, for example, Al2O3, HfO2, Ta2O5, and/or TiO2 that has a dielectric constant that is several times to a hundred times or more higher than the dielectric constant of a conventional oxide layer/nitride layer/oxide layer (ONO) capacitor dielectric layer.
Doped polysilicon layers have conventionally been used to form the electrodes of many semiconductor capacitors. Doped polysilicon, however, may react with the materials that are used to form to a high dielectric constant dielectric layer, and such reactions may deteriorate the electrical characteristic of the capacitor. As such, various alternative electrode materials have been investigated for use in capacitors having high dielectric constant dielectric layers. One example of such an alternative electrode material is a stacked layer consisting of (1) a metal, a conductive oxide of a metal, or a conductive nitride of a metal layer and (2) a doped polysilicon layer. The metal layer, the conductive metal oxide layer or the conductive metal nitride layer may all exhibit a small reactivity with the high dielectric layer. The doped polysilicon layer may be formed by depositing polysilicon via a low pressure chemical vapor deposition (LPCVD) process performed at, for example, about 530° C., and then performing a thermal treatment at, for example, a temperature of greater than about 600° C. for 30 minutes in an N2 atmosphere. However, the leakage current characteristic of the device may deteriorate as a result of the high temperature thermal treatment.
Another alternative electrode material is a stacked layer consisting of a metal layer and a doped polysilicon germanium layer. The doped polysilicon germanium layer can be formed by a low temperature process. However, the stacked metal layer and doped polysilicon germanium layer electrode may exhibit increased leakage currents due to diffusion of the dopants in the polysilicon germanium layer into lower layers of the capacitor.